An extension B⊂A of algebras over a commutative ringk is an H-extension for an L-bialgebroid H if A is an H-comodule algebra and B is the subalgebra of its coinvariants. It is H-Galois if the canonical map A ⊗BA →A ⊗LH is an isomorphism or, equivalently, if the canonical coring (A ⊗LH:A) is a Galois coring. In the case of Hopf algebroid H=(HL, HR, S) any HR-extension is shown to be also an HL-extension. If the antipode is bijective then also the notions of HR-Galois extensions and of HL-Galois extensions are proven to coincide. Results about bijective entwining structures are extended to entwining structures over non-commutative algebras in order to prove a Kreimer-Takeuchi type theorem for a finitely generated projective Hopf algebroid H with bijective antipode. It states that any H-Galois extension B⊂A is projective, and if A is k-flat then already the surjectivity of the canonical map implies the Galois property. The Morita theory, developed for corings by Caenepeel, Vercruysse and Wang is applied to obtain equivalent criteria for the Galois property of Hopf algebroid extensions. This leads to Hopf algebroid analogues of results for Hopf algebra, extensions by Doi and, in the case of Frobenius Hopf algebroids, by Cohen, Fishman and Montgomery.
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